Finely dice the onions, quarter the mushrooms, fry both in hot oil for 5 minutes. Pour in the broth, bring to the boil and pour over the potatoes. Season with mustard, vinegar, salt and pepper, let cool, stir frequently.
Grate the zucchini, cut the radishes into slices, boil the peas in salted water for 2 minutes, add everything to the salad.
Quarter and core the tomatoes, cut into strips and stir in just before serving.